Vending Machine

Prompt

"Design a vending machine. Customers put in money, pick a product, get the product and change. Show me how you model the machine's states and what happens on each input."

Clarifying questions

• What payment forms? Coins and notes in fixed denominations. Card payment is a follow-up.
• Can a customer insert money and then cancel? Yes; the machine refunds the inserted amount.
• What if the exact change cannot be made? The machine should refuse the sale before dispensing and refund. Assume the machine's coin inventory is tracked.
• What if a product is out of stock? Refuse the selection, stay in the "has money" state so the customer can pick something else or cancel.
• Multiple customers at once? No; a physical machine serves one customer at a time. But an operator restocks and an API reads inventory, so some concurrency exists.
• Does the machine need to persist state? Money and inventory should survive a power cycle. Mention it, model it as a hook.

Requirements

Functional

• Accept money in valid denominations and show the running balance.
• Select a product by slot code; dispense it when the balance covers the price and change can be made.
• Return change in coins from the machine's own inventory.
• Cancel at any point before dispensing, refunding the balance.
• Operator functions: restock a slot, load coins, collect cash.

Non-functional

• Illegal sequences (select before paying, insert during dispensing) must be rejected, not silently ignored or half-applied.
• Every transition must leave the machine in a consistent state even if dispensing fails mid-way.
• Adding a state (out of service, maintenance) or a payment method must not require editing every existing operation.

Numbers

• Perhaps 30 slots, a coin inventory of a few hundred coins across 5 denominations, one transaction every few minutes. Performance is irrelevant; this problem is graded entirely on correctness of transitions and on structure.